In fuel cells, water is produced by a cell reaction as a product in principle. Fuel cells have therefore drawn attention as clean power generation systems without a substantially harmful influence on the earth's environment. For example, a polymer electrolyte fuel cell comprising a pair of electrodes on both sides of a polymer electrolyte membrane that conducts protons produces electromotive force by supplying hydrogen gas as a fuel gas to one of the electrodes (i.e., the fuel electrode: anode), and supplying oxygen gas or air as an oxidant to the other electrode (i.e., the air electrode: cathode).
The cell characteristics of polymer electrolyte fuel cells have been drastically improved by advances such as the following: (1) a polymer electrolyte membrane having high ion conductivity has been developed; and (2) catalyst-supported carbon coated with the polymer electrolyte consisting of a material that is the same as or different from that of the polymer electrolyte membrane is used as the constituent material of the electrode catalyst layer to form what is called a three-dimensional reaction site in the catalyst layer. In addition to the excellent cell characteristics described above, the polymer electrolyte fuel cell can readily be made smaller and lighter. Due to the characteristics described above, the polymer electrolyte fuel cell is expected to be put in practical use as a power source for mobile vehicles such as electrically powered cars or power sources for small cogeneration systems.
In general, the gas diffusion electrode used in a polymer electrolyte fuel cell consists of a catalyst layer, which contains catalyst-supported carbon materials coated with the polymer electrolyte, and a gas diffusion layer, which not only supplies the reaction gas to the catalyst layer but also collects electrons. The catalyst layer has open areas consisting of micropores formed among secondary or tertiary carbon particles, which are constituents of the catalyst layer, and the open areas function as diffusion channels of the reaction gas. As such catalysts, noble metal catalysts, such as platinum or a platinum alloy, that are stable in a polymer electrolyte are generally used.
In the past, a polymer electrolyte fuel cell involved the use of catalysts comprising a noble metal, such as platinum or platinum alloy, supported on carbon black as cathode and anode catalysts of the electrode catalysts. In general, platinum-supported carbon black is prepared by adding sodium bisulfite to an aqueous solution of platinic chloride, allowing the mixture to react with a hydrogen peroxide solution, preparing the carbon black particles to support the resulting platinum colloids, washing the resultants, and heating the resultants as needed. Electrodes of a polymer electrolyte fuel cell are prepared by dispersing platinum-supported carbon black particles in a polymer electrolyte solution to prepare an ink, coating the gas diffusion substrate, such as a carbon paper, with the ink, and drying the substrate. The polymer electrolyte membrane is sandwiched between such two electrodes, followed by a hot press. Thus, an electrolyte membrane-electrode assembly (MEA) can be constructed.
Platinum is an expensive noble metal, and it is thus expected to exhibit satisfactory performance by a small amount thereof. Accordingly, work is proceeding with catalyst activity in smaller amounts of platinum. For example, JP Patent Publication (kokai) No. 2002-289208 A is intended to provide an electrode catalyst for a fuel cell having high durability by inhibiting growth of platinum particles during operation, and discloses an electrode catalyst comprising a conductive carbon material, metal particles supported thereon that are less likely to be oxidized than platinum under acidic conditions, and platinum covering the outer surface of the metal particles. Specifically, the publication exemplifies an alloy comprising platinum and at least one metal selected from among gold, chromium, iron, nickel, cobalt, titanium, vanadium, copper, and manganese as the metal particle.
JP Patent Publication (kokai) No. 2002-15744 A is intended to provide a polymer fuel cell that has excellent cathode polarization properties and produces a high cell output, and disclose the catalyst layer of the cathode containing a metal catalyst selected from the group consisting of platinum and platinum alloy and a metal complex containing a given amount of iron or chromium to improve cathode polarization properties. Specifically, a polymer electrolyte fuel cell comprises an anode, a cathode, and a polymer electrolyte membrane located between the anode and the cathode, wherein the cathode comprises a gas diffusion layer and a catalyst layer located between the gas diffusion layer and the polymer electrolyte layer, the catalyst layer contains a noble metal catalyst selected from the group consisting of platinum and platinum alloy and a metal complex containing iron or chromium, and the content of the metal complex is 1 to 40 mol % of the combined quantity of the metal complex and the noble metal catalyst.
Moreover, JP Patent Publication (Kokai) No. 63-12349 A (1988) discloses obtainment of a platinum alloy catalyst by causing a carrier to support platinum thereon, adhering a metal compound for alloy formation to the carrier, reducing the metal compound, and then performing heat treatment for alloy formation.
Meanwhile, JP Patent Publication (Kokai) No. 2005-135900 A discloses an electrode catalyst for a fuel cell having a core-shell structure, in which both the core and the shell contain platinum.